Esters of organic acids are commercially extremely useful for a variety of purposes. Lower esters are extensively used as solvents in coatings, inks and adhesives, and in processing other substances. They readily dissolve resins or their precursors to become vehicles for application of coatings, etc.
Esters are also widely used as plasticizers in polymers. These esters include the benzoates, phthalates, terephthalates, and trimetillates, and aliphatic dibasic acid esters. Varying the acid or the alcohol components modifies the efficacy of the resultant ester as a plasticizer. For example, phthalate plasticizers, the molecular sizes of the alcohol moiety can be varied from methyl to tridecyl to control permanence, compatibility, and efficiency; branched (e.g., 2-ethylhexyl, isodecyl) for rapid absorption and fusion, or linear (C6-C11) for low temperature flexibility.
Unsaturated and difunctional ester are important monomers for the manufacture of many commercial polymers. For example, polymerization of vinyl acetate and methyl methacrylate produces poly(vinyl acetate) and methyl methacrylate. Another example is dimethyl terephthalate, which reacts with ethylene glycol to yield poly)ethylene terephthalate).
Monohydric alcohol esters of dibasic acids and polyol esters of monobasic acids are used as synthetic lubricants. They are generally prepared from C8-C13 monohydric alcohols, polymethylol compounds such are trimethylolpropanes, pentaerythritol, and dipentaerythritol; C6-C10 monobasic acids such as heptanoic and nonanoic acids, and C6-C10 dibasic acids such as adipic, azelaic and sebacic acids, and phthalic anhydride. These esters are mainly used as base oils in high performance lubricants for engines and machinery.
Esters in the form of fats and oils from tallow and plants such as soybean, cottonseed, linseed and castor bean are important raw materials for soap, paints and food industries.
Polyol (e.g., glycerol, sorbitol, sucrose and propylene glycol or poly(ethylene oxide) esters of long-chain fatty acids are nonionic surfactants used in foods, pharmaceuticals, cosmetics, textiles, cleaning compounds, and many other applications.
Many esters are used as pharmaceuticals. Of these, benzocaine, ethyl 4-aminobenzoate, is a topical anesthetic. Phenyl salicylate has antipyretic, antirheumatic, and antiparasitic properties. Some simple benzoates are also used as antiseptic agents. Salicylic acid esters are used as antibacterial agents and pain relievers. Analgesic balms, creams, sprays, and nasal inhalers usually contain various combinations of either methyl or menthyl salicylate and menthol.
In general, esterification of a physiologically active alcohol or phenol with aliphatic carboxylic acid or an acid with alcohol detoxifies it by decreasing the concentration of thee active component present. The active compound is released gradually in the body by hydrolysis of the ester. Examples of these include aspirin, a common analgesic; methyl phenidate, a central nervous system stimulate; and clofibrate, an antihyperlipoproteinemic.
Many parent drugs have been converted to esters to generate so-called prodrugs in order to overcome some undesirable property, such as bitter taste, poor absorption, poor solubility, and irritation and injection site. For example, antibiotics such as chloramphenicol and clindamycin have been derivatized as their palmitate esters in order to minimize their bitter taste.
In order to improve the poor oral absorption of carbencillin, a lipophilic indanyl ester has been produced. Prednisolone, a steroid, is derivatized to its C21 hemisuccinate sodium salt to make it extremely water-soluble.
Several esters are used as herbicides and pesticides. Many halogenated benzoic acid esters are phytotoxic and are used as herbicides. Demthyltetrachloroterephthalate and diisopropyl 5-nitroisophthalate are used as herbicide and fungicide, respectively. The phenoxy herbicides are primarily propyl, butyl and isooctyl esters of 2, 4-dichlorophenoxyacetic acid, 4-chloro-2-methylphenoxyacetic acid, and methyl, ethyl or butyl esters of 2-(4-hydroxyphenoxy)propionic acid. Because of their low toxicity, high selectivity, and relatively short life in the soil, phenoxy herbicides are widely used. They are used for controlling weeds in a large number of grass crops such as corn, small grains, sorghum, rice, sugarcane, pasture, range land and turf.
Pyrethroids are synthetic esters produced to imitate or improve the activity of biological principals of the pyrethrum plant. They are powerful contact insecticides, causing rapid knockdown of treated insects. The pyrethroids are extensively used in controlling insect pests on fruit trees, vegetables, and other field crops in space sprays and contact sprays to kill insects infesting homes, industrial locations, and nonfood processing areas; and in protection of warehoused food. These compounds include fenvelerate, flucythrinate, allethrin, cyfluthrin, deltamethrin, permethrin, and tetramethrin.
Since esters can be easily hydrolyzed, they are used as protecting groups for hydroxyl and carboxylic acid groups. Acetates and benzoates are widely used in carbohydrate, steroid and nucleoside chemistry, and their cleavage is based on hydrolysis with base, ammonolysis, or methanolysis. Of great importance in peptide chemistry are the t-butyl, benzyl, and substituted benzyl esters.
Esters are conventionally produced by reacting an alcohol with an organic acid in the presence of a catalyst, or by transesterification of an ester in which the alcohol group of the ester is replaced by another alcohol group.
One example of such transesterification is the preparation of ethyl esters of fatty acids from triglycerides. In this process, a triglyceride is mixed with absolute ethanol and catalytic amounts of sodium or potassium methoxide. The mixture is then left for at least twelve hours, or overnight, and then washed with water to remove the glycerin, ethanol, and sodium or potassium methoxide.
One widely used catalyst for preparation of esters is 4-dimethylaminopyridine (DMAP). This catalyst can be used for acid-alcohol reactions or for transesterification reactions. However, the reaction times are measured in hours.